In a conventional wireless cellular telephone network, base transceiver stations (BTS) facilitate communication between mobile units (e.g. cellular telephones) and the telephone network. A typical base station comprises multiple transceiver units and antennas for sending radio signals to the mobile units (downlink) and for receiving radio signals from the mobile units (uplink). The antennas of the base station are positioned outdoors on a cellular tower or on top of a building. Base stations are strategically located so as to maximize communications coverage over large geographical areas. The base stations are communicatively coupled to the telephone network via backhaul connections.
Conventional base stations have employed one transmit output signal and one receive signal for each antenna of the base station. The transmit and receive signals operate at different frequencies which allows for separation of the transmit and receive signals. This allows the transmit and receive signals to be combined by a duplexer so that a single antenna can be used for transmitting and receiving these two signals. So that multiple mobile devices can communicate simultaneously via the antenna, the transmit and receive signals are each divided into plural channels. For example, CDMA (Code-Division, Multiple Access) protocols for cellular communication, such as UMTS (Universal Mobile Telecommunications System), and TDMA (Time-Division, Multiple-Access) protocols for cellular communication, such as GSM (Global System for Mobile Communications), allow several communications channels to be combined into the transmit and receive signals.
In another configuration, a base station employs receive diversity in which two antennas are used for receiving the uplink signal from the mobile units. In this case, the receive antennas are co-located in that they are mounted to the same outdoor structure but spaced slightly apart from each other. The signals from each antenna are then combined to form the uplink signal. This configuration can increase the cell coverage or allow the mobile units to use a lower output power.
Newer generations of base stations support multiple-input, multiple-output (MIMO) configurations. One example is MIMO 2x2 in which the base station uses two transmit antennas for the downlink signal and two receive antennas for the uplink signal. Another example is MIMO 4x4 in which four transmit antennas are employed for the downlink signal and four receive antennas are employed for the uplink signal. In all these MIMO configurations, the antennas are co-located in that they are mounted to the same outdoor structure but spaced slightly apart from each other
These MIMO configurations are used to improve the throughput of a base station. This is achieved by the base station combining the multiple signals obtained from the multiple antennas to make the combined signal stronger or to reduce interference.
While these MIMO configurations can improve communications when the mobile devices are located outdoors, signal attenuation caused by building structures (i.e. indoor penetration loss) can still result in communication difficulties for mobile units when located indoors.
In a distributed antenna system (DAS), transmitted power is divided among several antennas in distributed locations so as to provide a large coverage area using less transmitted power than would be required by a single antenna. DAS systems have been used to provide indoor coverage for wireless communications. However, conventional distributed antenna systems leave room for improvement.
What is needed is an improved distributed antenna system for wireless communications.